Solid-state image capture devices such as electronic still cameras and the like rely on an array of discrete light sensitive elements or photosites known as pixels to spatially sample an image of the object and produce sampled output. Because the image is sampled in a discrete manner, images re-created contain false signal content not present in the original object. This phenomenon is known as aliasing and becomes noticeable for those spatial frequency components of the image which are equal to half the spatial sampling frequency along a particular direction of the pixel array, the so-called Nyquist frequency. Although aliasing begins to appear at the Nyquist frequency, it is most pronounced at spatial frequencies that are close to the sampling frequency along the given direction of the pixel array. One particular optical filter, the phase noise filter, is designed to remove the unwanted higher frequencies above the Nyquist frequency of such imagers.
A typical plot of MTF vs. spot height, at the sampling frequency, for three wavelength ranges, for a phase noise filter made of a material having a refractive index of approximately 1.55, is shown in FIG. 1. Color selective phase noise filters are designed to allow the green wavelength band 12 of light (500-570 nm) to pass through the filter with minimum degradation to the MTF, while the MTFs for the blue 13 (400-500 nm) and red 14 (570-700 nm) wavelength bands are reduced to a minimum, when the imaging system uses a Bayer pattern color filter array. As a result of these requirements, a color selective phase noise filter made with a material having a refractive index of approximately 1.55 will have a thickness of approximately 2.86 microns. Non-color selective phase noise filters are designed to have minimum MTF over the three wavelength ranges (400-700 nm), and therefore will have a thickness of approximately 0.4 microns.
In prior art, phase noise filter applications, both color selective and non-color selective as described in the above referenced patent applications, the disclosures of which are incorporated herein by reference, the filters were designed to have a random array of spots covering between 25% and 50% of the filter surface, with no spots on the remaining 75% to 50%. This "fill factor" or "duty cycle" was chosen to maximize blurring of the unwanted light frequencies above the Nyquist point of the sensor. The term "fill factor" means the percentage of the total area of the spots divided by the total filter area. A diagram of a typical random spot pattern of mono-dispersed circular spots is shown in FIG. 2. The same effect holds when the spot diameters are varied about a mean diameter, while maintaining the fill factor.